With increasing development of information industries and high-tech industries, most precise electronic instruments and facilities need highly reliable power to maintain normal operations. Generally, an uninterruptible power supply (UPS) are widely used to provide stable power to the loads that are connected with. In other words, UPS apparatuses become essential for supplying stable power.
The conventional uninterruptible power supply principally includes an AC-to-DC converting circuit, a DC-to-AC converting circuit and a bus capacitor. The output terminal of the AC-to-DC converting circuit is connected to the bus capacitor and the input terminal of the DC-to-DC converting circuit. By the AC-to-DC converting circuit, an AC input voltage is converted into a regulated DC voltage and a bus voltage is generated to the DC bus. The bus capacitor is connected to the DC bus for filtering off undesirable noise contained in the bus voltage, thereby providing a constant bus voltage. By the DC-to-AC converting circuit, the constant bus voltage is converted into an output AC voltage required for powering a load.
That is, if the input AC voltage is suffered from a sudden variation or interruption, the uninterruptible power supply apparatus can still generate a stable and uninterruptible output AC voltage to the load and thus a high-quality output AC voltage is continuously transmitted to the load.
Moreover, since the amount of electric energy outputted from the AC-to-DC converting circuit and the duty cycle of the internal switching circuit are adjusted, the output voltage of the AC-to-DC converting circuit (i.e. the bus voltage) can be maintained at the rated voltage value.
When the conventional uninterruptible power supply is initiated or enabled, the voltage across the bus capacitor and the electric energy stored in the bus capacitor are zero. As a consequence, the voltage difference between the bus voltage and the rated voltage value is very huge and internal switching circuit is adjusted to be operated at the maximum duty cycle. At the same time, the AC-to-DC converting circuit generates a very huge current to charge the bus capacitor. Since a huge initiation current is generated in the input side of the uninterruptible power supply, an inrush current is ready generated.
For preventing the uninterruptible power supply from being burnt out, the input side of the uninterruptible power supply is usually connected to a NTC (negative temperature coefficient) thermistor in series. Due to the inherent property of the thermistor, the initiation current generated when the uninterruptible power supply is initiated will be reduced. In some cases, a phase-adjusting circuit including a silicon-controlled rectifier (SCR) is connected to the input side of the uninterruptible power supply. The phase-adjusting circuit is controlled by a phase control circuit. As a consequence, the input AC voltage with certain phases can be transmitted to the AC-to-DC converting circuit through the phase-adjusting circuit to reduce the initiation current.
Although the thermistor is effective for reducing the initiation current, there are still some drawbacks. For example, the thermistor may consume additional power and thus the overall efficiency of the uninterruptible power supply is decreased. Moreover, the resistance value of the thermistor needs to be adjusted according to the capacitance value of the bus capacitor. Since there is a large error between the practical capacitance value and the labeling capacitance value of the bus capacitor, the resistance value of the thermistor is usually undesired. Under this circumstance, the efficacy of using the thermistor to reduce the initiation current is insufficient.
Moreover, the phase-adjusting circuit can only adjust the phase of the input AC voltage to be transmitted to the AC-to-DC converting circuit. The phase-adjusting circuit, however, fails to adjust the magnitude of current during the on phase period. As known, the magnitude of current generated during the on phase period is varied depending on the magnitude of the input AC voltage. After the silicon-controlled rectifier of the phase-adjusting circuit is conducted, the silicon-controlled rectifier of the phase-adjusting circuit fails to be shut off under control of the phase control circuit and thus the initiation current can not be accurately reduced. Since the phase control circuit is very complicated, the stability of the phase control circuit is usually insufficient. In other words, the overall stability of the uninterruptible power supply is unsatisfactory.
Therefore, there is a need of providing an improved uninterruptible power supply to obviate the drawbacks encountered from the prior art.